This invention is directed generally to an apparatus and method for cooling materials, and specifically to cooling a PVC compound.
Polyvinyl chloride, or PVC, is a well known chemical material which has widespread use in the manufacturing of various products, such as pipes, containers, and a large number of other products. In utilizing PVC in the manufacturing process, the PVC is often supplied and handled in bulk form as a loose powder or resin. The bulk material is subsequently heated to a melting point and molded into a finished product, such as through an injection molding process, for example.
Before being utilized in the molding process, raw PVC resin, must be mixed or compounded with other materials desirable for the particular product to be made or the particular end use of the PVC. For example, the raw PVC resin can be mixed with plasticizers, stabilizers, lubricants, and even pigments, such as for adding color to the PVC. Such compounding or mixing is generally accomplished in large industrial mixers which assure uniform dispersion of the plasticizers, stabilizers, and other components within the PVC resin powder while concurrently assuring that dry additives, such as dyes and lubricants impinge upon the resin particle surfaces. Commercial mixers for such purposes are available as the Model W Series from Littleford Day, Inc. of Florence, Ky., for example, which is the owner of the present application.
During compounding, the mixer converts the mixing energy of the process into a desirable uniform heat due to the particle contact friction in the batch of compounded material. The heat is necessary for the compounding process. However, in order to further process and store the PVC resin and to improve the flow characteristics of the PVC resin, the heat must then be removed from the batch. Therefore, processing of PVC resin involves a cooling step.
For cooling, large cooling units or coolers are used, and the bulk PVC resin is cooled from temperatures in the range of 220xc2x0-250xc2x0 F. (a heat over 212xc2x0 F. is usually desirable to reduce moisture in the mixture) to a range of 120xc2x0-125xc2x0 F. as a finish temperature. Industrial coolers utilize large mixing chambers with cooled walls against which the PVC resin is directed. In one such unit, cooling fluid, such as water, is circulated against the walls of the chamber to cool its inner surface. Mixing tools which are mounted to rotate on a horizontal shaft are used to create a fluidized action which constantly exposes new surfaces of the PVC resin to the heat exchange surface on the inside of the chamber. This creates a desirable high rate of heat transfer from the PVC resin to the chamber walls. The cooling chamber is then emptied of the cooled PVC from discharge ports. Alternatively, the cooling chamber might be incorporated into a continuous system, wherein the cooled and dried PVC compound is delivered directly to a conveyor. PVC cooling units are available as Model K Series from Littleford Day, Inc. of Florence, Ky.
While existing PVC cooling units or coolers are suitable for processing PVC, it is still desirable to improve on such coolers and to improve their efficiency and product throughput. Specifically, it is desirable to improve the heat transfer characteristics of the cooler, so that a particular batch of material may be cooled more rapidly. The shorter the batch cooling time, the greater the number of batches of material that may be cooled within a particular time interval. Therefore, improving the heat transfer characteristics of the cooler results in greater batch throughput and consequently greater processing revenues and reduced processing costs per batch.
In existing PVC coolers, metal mixing tools, such as stainless steel blades, or scrapers, are mounted to extend radially from a horizontal shaft which, in turn, extends through the mixer. As the shaft rotates, the scrapers or other tools sweep around the generally cylindrical chamber against the cooled inner cylindrical surface of the chamber. The PVC compound is thereby turned over, mixed, and otherwise directed against the cooled surface for heat transfer purposes. The tools continuously move the cooled PVC away so that a new layer of PVC is directed against the cooled surface.
The mixing tools rotate at a desirable speed for turning over the batch and presenting new material to the inner surfaces for cooling. In one embodiment, the shaft and tools normally rotate in the range of 80 to 130 rpm, depending on cross-sectional diameter of the chamber. Increasing the rotational speed of the tools does not necessarily provide more rapid cooling, however, because the mixing and sweeping action of the tools introduces energy, and therefore heat, into the process. The inventors have found that the increased heat energy from high rotation speeds limits the cooling benefit gained in existing prior art PVC coolers. Therefore, rotational speeds for the mixing tools have generally been confined to 80-130 rpm.
A further drawback with existing PVC coolers is that a boundary layer of PVC builds up between the inner cooling surface and the tools due to mechanical tolerances. This boundary layer at the cooling surface hinders the overall cooling process and the heat transfer at the inner chamber surface. More specifically, due to the expansion and contraction of the metal components of a PVC cooler, including the chamber, the rotating shaft and the mixing tools, a suitable clearance (such as xc2xc inch) between the outer edge of the metal mixing tools and the chamber inner surface must be maintained. The PVC becomes compacted in the clearance space and forms an insulative boundary layer which hinders the heat transfer at the surface and thus hinders cooling of the PVC resin.
Accordingly, it is an objective of the invention to improve the process of cooling a material, such as a PVC compound.
More specifically, it is an objective to reduce the batch cooling time to improve the throughput rate of a cooler or cooling unit.
It is still another objective to reduce boundary layers and build up of material at the cooling surface.
These objectives and other objectives will be more readily apparent from the description of the invention below.
An apparatus and method for improved processing of a material, such as a PVC compound utilizes a chamber having an inner surface for receiving a batch of material to be cooled. A rotatable shaft extends through the chamber inner space, and the shaft is operable for being rotated by a motor. At least one mixing tool is coupled to the shaft to rotate with the shaft, and is adapted to mix the material while rotating around the inner surface of the chamber so that the material contacts the inner surface, to be cooled by that inner surface. In one embodiment of the invention, a double-walled or jacketed chamber may be utilized with cooling fluid, such as water, circulated around the chamber for cooling purposes.
In accordance with one embodiment of the invention, the mixing tool comprises a flexible blade which is configured for contacting the chamber inner surface. The flexible blade is operable when the mixing tool is rotated, for sweeping across a portion of the inner surface and dislodging an amount of material from the surface portion so that another amount of material may contact the surface portion and be cooled. The motor which rotates the mixing tool and flexible blade is operable for rotating the shaft at a speed in the range of 150-250 rpm to sweep the material from the inner surface. The present invention thus provides improved cooling characteristics for cooling a material, such as a PVC compound.
In accordance with one aspect of the present invention, the flexible blade may be made of a PVC material, similar to a PVC material which is being cooled. That is, the flexible blade may be made of a material which is present in the material being cooled. One embodiment utilizes a flexible blade which is releasably secured with the mixing tool so that it may be readily replaced or exchanged. In accordance with another aspect of the present invention, the flexible blade includes a plurality of slots formed therein so that the blade resembles a comb-like structure.
In still another aspect of the present invention, the flexible blade may be formed of material having one of a number of selectable colors for matching the color of the material being cooled. In that way, the invention may experience blade wear during the cooling process, wherein any material worn from the blade matches the color of the material which is being cooled.
In accordance with another embodiment of the invention, the mixing tool comprises an air knife which is configured for directing a stream of pressurized air against the inner surface when the tool is rotated. The stream of air dislodges an amount of material from a portion of the surface so that another amount of material may contact the surface portion to be cooled. The stream of air might be directed perpendicular to the inner surface or tangential to the inner surface, or any angle therebetween. Pressurized air is delivered to the air knife through a hollow portion of the rotatable shaft, and the air of the air knife is appropriately vented from the chamber.
The present invention has significantly reduced the cooling times for a batch of material such as a batch of PVC compound. Therefore, the present invention addresses the objectives set forth above and other objectives, and provides an improvement over the prior art. Further details of some embodiments of the invention are set forth below.